Conventionally, superconducting cables including superconducting conductors formed from Bi-based high-temperature superconducting wires and the like are known. FIG. 2(A) is a cross-sectional view of a three-core type three-phase superconducting cable including three cable cores, and FIG. 2(B) is a perspective view illustrating an example of the core. A superconducting cable 100 is configured to include cabled three cable cores 102 enclosed within a heat-insulating pipe 101.
Referring to FIG. 2(A) and FIG. 2(B), heat-insulating pipe 101 has a configuration including a double pipe consisting of a corrugated outer pipe 101a and a corrugated inner pipe 101b and a heat-insulating material (not shown) disposed therebetween, the inside of the double pipe being vacuumed. Each cable core 102 includes, in order from the innermost thereof, a former 200, a superconducting conductor 201, an electric insulating layer 202, a shield layer 203, and a flaw-protecting layer 204. Former 200 is formed from a normal-conducting material such as copper or aluminum to be a hollow shape or a solid shape. Superconducting conductor 201 is formed by spirally winding superconducting wires on and around former 200 to be multiple layers. Electric insulating layer 202 is formed by wrapping an insulating material such as semi-synthetic insulating papers. Shield layer 203 is formed by spirally winding superconducting wires similar to superconducting conductor 201 on and around electric insulating layer 202. In normal conditions, there are induced, in shield layer 203, a current with substantially the same magnitude as that of a current flowing through superconducting conductor 201 in the direction opposite thereto. Magnetic fields created by such an induced current can cancel the magnetic fields created by superconducting conductor 201, thereby substantially nullifying magnetic fields leaking from cable core 102 to the outside. Generally, the space 103 defined by inner pipe 101b and respective cable cores 102 forms a refrigerant flow path. Further, on corrugated outer pipe 101a, there is formed a reinforcing layer (protective covering outer sheath) 104 made of polyvinyl chloride and the like.
In the event of accidents such as short-circuits or ground faults in the electric-power system for the superconducting cable, this will induce large currents therein. Therefore, there is a need for taking measures for suppressing fault currents such as the installation of a current-limiting device, because otherwise large currents exceeding steady-state currents will flow through the superconducting cable. For example, when the rated voltage is 350 MV and the rated current is 3 kA, a short-circuit current of about 31.5 kA/will be induced in the event of short-circuit accidents (in an exemplary line, a current of about 31.5 kA will flow for 1 second). When large currents exceeding the critical current value flow through the superconducting conductor, this superconducting conductor will be shifted (quenched) to a normal-conductor, and this shift will induce Joule losses (heat losses). Concurrently, large currents will be induced in the shield layer, which will shift the shield layer to a normal conductor, thus causing Joule losses. Particularly, when significant Joule losses are caused, this may cause burning of the superconductor wires constituting the superconducting conductor or the shield layer or otherwise may suddenly raise the temperature thereof to vaporize refrigerant trapped in voids within the wires, resulting in ballooning (nitrogen ballooning) of the superconducting wires and thus lowering the critical current value. Further, the vaporization of refrigerant may cause dielectric breakdown. In this case, it will require a significantly long time to repair damages caused by such accidents.
Therefore, there have been known techniques for providing a copper layer between the superconducting conductor and the electric insulating layer (see Patent Document 1) or for providing a copper layer on and around the outer circumference of the protective layer (see Patent Document 2), in order to divert currents into the aforementioned metal layers for suppressing heat generation in the superconducting layers, in the event of the occurrence of large currents due to accidents such as short-circuits. Also, Patent Document 3 describes a configuration including multiple shield layers and multiple copper layers provided on the outer circumference of the electric insulating layer, the shield layers being provided between the copper layers.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-067663 (see the claims and FIG. 1)
Patent Document 2: Japanese Laid-Open Patent Publication No. 2001-052542 (see the claims and FIG. 1)
Patent Document 3: Japanese Laid-Open Patent Publication No. 2002-008459 (see the claims and FIG. 1)